DESCRIPTION: (verbatim from application) The overall goal of this work is to develop more effective, physiologically based, non-operative rehabilitation programs to dynamically stabilize the knee after anterior cruciate ligament rupture. Some individuals can stabilize their knees following anterior cruciate ligament (ACL rupture even during activities involving cutting and pivoting while others have instability wit daily activities (non-copers). Physiological responses and motor control strategies for those who successfully compensate for the absence of the ACL are different from those who do not compensate well for the injury. A rehabilitation program that includes purposeful perturbation of support surfaces results in superior return to functional activity in those with good rehabilitation potential. Several questions remain that impact the development of effective rehabilitation programs for patients after ACL rupture. How does the training promote dynamic knee stability? Can adding perturbation training to a vigorous rehabilitation program improve function in non-copers who elect non-operative management? The proposed experiments are designed to answer these questions. Aim 1, to elucidate the mechanism underlying the development of dynamic stability of the knee as a result of perturbation training in patient with ACL deficiency. A platform that, when unlocked, translates anteriorly immediately after initial contact will be used to destabilize the knee during walking. Will be carried out in a population of 20 ACL deficient individuals classified as rehabilitation candidates and 20 matched uninjured subjects. Aim 3, to compare treatment effectiveness in restoring knee stability in non-copers between a standard non-operative anterior cruciate ligament rehabilitation program and one that is augmented with perturbation training techniques, a randomized, blinded, controlled clinical trial will be carried out in a population of 46 ACL deficient non-copers.